この問題を解決するための私の戦略は、いくつかのレベルの間接参照を使用することです。
- zipper <Args ...>は、次を継承することにより、引数の処理を関数process_zipper_argumentsにディスパッチします。
例：
template < typename... Args >
struct zipper : zipper < typename process_zipper_arguments < Args... >::type > {};
- を使用し
template < typename... Args > struct typelist {}て、継承するオブジェクトタイプを追跡します。
struct zipper < typelist < Args... > >: public virtual Args...実際の継承を行うことを専門とする
重複する親タイプを取り除くために、2つのヘルパー関数が次の場所で使用されprocess_zipper_argumentsます。
is_in < CandidateType, typelist< Args... > >::typeまたはのいずれtrue_typeかfalse_typeであり、再帰的に定義できますadd_unique < CandidateType, typelist< Args... > >::typeCandidateTypeがtypelist <...>追加されているかどうかに関係なくです。is_inそれを決定するために呼び出します。
これは、少なくともg ++（GCC）4.6.3と--std = c++0xでコンパイルされる完全なコードです。それに対する批判は大歓迎です。
// Forward declarations
template < typename... Args >
struct zipper;
// Two types meaning true and false
struct true_type {};
struct false_type {};
// The only purpose of this struct is to be associated with Types...
template < typename... Types >
struct typelist {};
// ===================================================
// is_in < type, typelist<...> >::type
// is true_type if type is in typelist
// is false_type if type is not in typelist
// Assume TElement is not in the list unless proven otherwise
template < typename TElement, typename TList >
struct is_in {
typedef false_type type;
};
// If it matches the first type, it is definitely in the list
template < typename TElement, typename... TTail >
struct is_in < TElement, typelist < TElement, TTail... > >
{
typedef true_type type;
};
// If it is not the first element, check the remaining list
template < typename TElement, typename THead, typename... TTail >
struct is_in < TElement, typelist < THead, TTail... > >
{
typedef typename is_in < TElement, typelist < TTail... > >::type type;
};
// ===================================================
// add_unique < TNew, typelist<...> >::type
// is typelist < TNew, ... > if TNew is not already in the list
// is typelist <...> otherwise
// Append a type to a type_list unless it already exists
template < typename TNew, typename TList,
typename Tis_duplicate = typename is_in < TNew, TList >::type
>
struct add_unique;
// If TNew is in the list, return the list unmodified
template < typename TNew, typename... TList >
struct add_unique < TNew, typelist < TList... >, true_type >
{
typedef typelist < TList... > type;
};
// If TNew is not in the list, append it
template < typename TNew, typename... TList >
struct add_unique < TNew, typelist < TList... >, false_type >
{
typedef typelist < TNew, TList... > type;
};
// ===================================================
// process_zipper_arguments < Args... >::type
// returns a typelist of types to be inherited from.
//
// It performs the following actions:
// a) Unpack zipper<...> and typelist <...> arguments
// b) Ignore values that are already in the list
template < typename... Args >
struct process_zipper_arguments;
// Unpack a zipper in the first argument
template < typename... ZipperArgs, typename... Args >
struct process_zipper_arguments < zipper < ZipperArgs... >, Args... >
{
typedef typename process_zipper_arguments < ZipperArgs..., Args... >::type type;
};
// Unpack a typelist in the first argument
template < typename... TypeListArgs, typename... Args >
struct process_zipper_arguments < typelist < TypeListArgs... >, Args... >
{
typedef typename process_zipper_arguments < TypeListArgs..., Args... >::type type;
};
// End the recursion if the list is empty
template < >
struct process_zipper_arguments < >
{
typedef typelist < > type;
};
// Construct the list of unique types by appending them one by one
template < typename THead, typename... TTail >
struct process_zipper_arguments < THead, TTail... >
{
typedef typename
add_unique < THead,
typename process_zipper_arguments < TTail... >::type
>::type type;
};
// ===================================================
// The zipper class that you might want
// If the list of types is not yet known, process it.
// The inheritance is ugly, but there is a workaround
template < typename... Args >
struct zipper : zipper < typename process_zipper_arguments < Args... >::type >
{
// // Instead of inheriting, you can use zipper as a factory.
// // So this:
// typedef zipper < meas2, zipper < meas1, meas > > mymeas;
// // Turns to:
// typedef typename zipper < meas2, zipper < meas1, meas > >::type mymeas;
typedef zipper < typename process_zipper_arguments < Args... >::type > type;
};
// If the list of types is known, inherit from each type
template < typename... Args >
struct zipper < typelist < Args... > >
: public virtual Args...
{};
// ===================================================
// Short usage demo, replace with your own code
struct meas {
int i;
};
struct meas2 {
int j;
};
struct meas3 {
int k;
};
typedef zipper < meas, meas, meas3 > meas_type;
typedef zipper < meas2, meas_type, meas2 > meas_type2;
typedef typename zipper < meas_type2 >::type nicer_meas_type2;
int main ( int, char** )
{
meas * m = new meas_type2;
meas_type2 n;
nicer_meas_type2 o;
return 0;
}
デバッグすると、次の結果が得られます（return 0;行のブレークポイント）。
(gdb) print *m
$1 = {i = 0}
(gdb) print n
$2 = {<zipper<typelist<meas, meas3, meas2> >> = {<meas> = {i = 4196320}, <meas3> = {k = 0}, <meas2> = {j = 0},
_vptr.zipper = 0x400928}, <No data fields>}
(gdb) print o
$3 = {<meas> = {i = 4195719}, <meas3> = {k = 0}, <meas2> = {j = 1}, _vptr.zipper = 0x4009a8 <VTT for zipper<typelist<meas, meas3, meas2> >>}